Credit America, turning up communist campaigner (8)
I believe the answer is:
crusader
'campaigner' is the definition.
(I've seen this before)
'credit america turning up communist' is the wordplay.
'credit' becomes 'cr'.
'america' becomes 'usa' ('U.S.A.' can be a synonym of 'america').
'turning up' says the letters should be written backwards.
'communist' becomes 'red' (red is a kind of communist).
'red' written backwards gives 'der'.
'cr'+'usa'+'der'='CRUSADER'
(Other definitions for crusader that I've seen before include "RA cursed (anag) - one fighting for a cause" , "religious warrior" , "What Simon de Montfort became" , "An old soldier" , "religious zealot" .)